1. Field of the Invention
The present invention relates to a communication apparatus and method for preventing interference with information transmitted from another communication apparatus.
2. Description of the Related Art
Radio tags or RF-ID tags for identifying objects have attracted attention as replacements for bar codes.
In the near future, a so-called ubiquitous society will be realized in which all objects are assigned a radio tag or RF-ID tag.
Japanese Patent Laid-Open No. 2002-162918 describes a poster having a radio tag or an RF-ID tag embedded therein. When a user who sees the poster wants information of the content written in this poster, the user directs his/her receiver to the radio tag or RF-ID tag embedded in the poster to obtain ID information from the poster, and then acquires the desired information via the Internet.
However, in a society where every object contains a radio tag or RF-ID tag, users would be surrounded by a plurality of radio tags or RF-ID tags. When a user directs a receiver to the above described poster in order to acquire information from the poster, a plurality of radio tags or RF-ID tags from objects in the vicinity of the poster emit their ID information in response to an ID acquisition instruction signal issued by the receiver, thus interfering with the information the user wants.
One approach to prevent interference in acquiring ID information is using a receiver with a highly directional antenna. However, this approach is problematic in that it is inconvenient for a user to direct the antenna or the receiver itself to a radio tag or an RF-ID tag each time the user wants to acquire information. The “Smart Active Label Consortium,” which was established in November 2002, has plans to provide internal batteries for radio tags or RF-ID tags so as to increase the ID information output power. As such radio tags or RF-ID tags that provide stronger transmission power will become wide-spread, thereby increasing the likelihood of interference between the various transmissions of ID information.
Japanese Patent Laid-Open No. 8-161653 describes a locator for lost objects in which a transmitter (e.g., radio tag or RF-ID tag) selects one of a plurality of frequencies (e.g., f1 to fn) and sends one ID information, including a pre-sync signal, to a receiver. The receiver receives the same ID information a plurality of the times and synchronizes the pre-sync signals and received one ID information. The goal of this invention is to ensure receipt of ID information from a specific apparatus without interference by ID information transmitted by different apparatuses by selecting the sending and receiving frequency.
The locator described in Japanese Patent Laid-Open No. 8-161653 is not useful in a very crowded place, such as an airport. In a crowded place where a plurality of radio waves can be exchanged, when a source of strong radio waves and a desired transmitter, each having different IDs, are near each other, the strong radio waves can cancel the transmission waves of the transmitter. Therefore, a user would not able to receive the ID information of the desired transmitter. Since the transmission frequency is fixed so as to coordinate with a receiver, the locator described in this publication is not suitable for an application to acquire ID information from multiple radio tags or RF-ID tags at the same time.
The locator described in Japanese Patent Laid-Open No. 8-161653 has a problem in that, in a case where a user possesses a plurality of objects which the user does not want to leave behind or lose, the user must either carry a plurality of receivers or carry a single receiver having a receiving-frequency changeover switch, which must be switched each time a plurality of objects are to be located.
The system described in Japanese Patent Laid-Open No. 8-335238 uses a locating system disclosed in U.S. Pat. No. 5,214,410 entitled “Location of objects” (i.e., so-called “Supertag” technology) to prevent interference.
In this system, a transmitter includes an internal clock generator, having the functions of using an ID transmission instruction from a receiver as a timing signal and count clock thereof in synchronization with the timing signal. The transmitter has one number stored in a read-only memory (ROM), and compares the clock count with the stored number. When a match is found, one-time transmission of ID information is carried out.
Each radio tag or RF-ID tag has a unique delay time, and has the function of transmitting information after the delay time has elapsed. If a plurality of radio tags or RF-ID tags exists, these tags do not transmit information, such as ID information, at the same time, thus preventing interference.
However, when radio tags or RF-ID tags that transmit ID information at the same time are accidentally located near each other, interference occurs. Interference is created, even if a receiver that detects an error sends an ID information re-transmission instruction, because the ID information is sent again at the same time. In other words, an interference error occurs again.
Another problem with the “Supertag” system is that one-time transmission of ID information causes a high probability of reception errors in a high-noise environment.
Moreover, since each tag contains a phase-locked loop (PLL) circuit that internally generates a timing signal and that synchronizes the generated timing signal with an ID transmission instruction, the chip size and the cost increase.